Traveling wave tubes are well known vacuum electron tubes that are used as final amplifier stages in broadband microwave transmitters. Traveling wave tubes incorporate an electron gun assembly having cathode and grid electrodes which coact with an anode electrode, and a helical radio frequency (r.f.) structure, inter alia. The electron gun generates a cylindrical electron beam, which is switched on and off by the grid electrode that is located closely adjacent to the cathode. The electron beam interacts with the field of a wave propagated along the helix with a velocity close to that of the electron beam. The electron beam has a velocity slightly greater than that of the propagated wave so that the beam is slowed by the field of the wave. The loss of kinetic energy of the electrons in the beam appears as an increase of energy in the field of the wave, thereby providing amplification of the signal being processed through the traveling wave tube.
Generally, in a traveling wave tube, the cathode is supported by a brazed stack of ceramics and metal cups, which are formed by machining or punching. The assembly is housed in a ceramic shell which is at high voltage so that it must be potted with a room temperature vulcanizing rubber (RTV) or other high voltage potting compound to prevent voltage breakdown. However, heat that is dissipated and radiated from the cathode heater flows through the high voltage ceramics assembly and causes the RTV to fail when overheated, resulting in voltage breakdown.
In addition, such prior art traveling wave tubes employ a large number of machined or punched parts that are assembled by welding and heliarcing processes and also include precision ceramic parts that require metallizing and brazing. The several parts and subassemblies of the tube need to be accurately positioned and aligned with jigs and by manual techniques. It is apparent that these prior art traveling wave tubes are very costly to manufacture and assemble and that reliability is adversely affected by the relatively complex assemblies.